1. Field of the Invention
The present invention relates to a sheet conveying apparatus for conveying sheets one by one, and to an image forming apparatus or an image reading apparatus having such a sheet conveying apparatus, and more particularly, it relates to an arrangement for monitoring a conveying condition of sheets being continuously conveyed.
2. Related Background Art
As an example, a conventional image forming apparatus or image reading apparatus includes a sheet conveying apparatus comprising a sheet feeding portion for feeding out a sheet and a conveying portion for conveying the sheet fed out from the sheet feeding portion to an image forming portion or an image reading portion. In such a sheet conveying apparatus, it is required that a conveying condition of the sheet be monitored to ensure that the function of the image forming apparatus and the like having the sheet conveying apparatus is satisfied and that serious damage is avoided.
To this end, in the conventional sheet conveying apparatuses, for example, as shown in FIG. 32 which is a schematic view of the conveying portion, there is provided a sensor 104 of flag type for detecting a sheet 105 being shifted in a conveying direction e to monitor a condition that the sheet 105 is being conveyed through a sheet conveying passage, and, sheets 105 being conveyed continuously with predetermined or more interval (sheet interval) are detected by the sensor 104 of flag type. In other words, by detecting the sheet interval by means of the sensor 104 of flag type, the conveying condition of the sheet 105 is monitored.
Incidentally, FIG. 33 shows a construction of such a sensor 104 of flag type. In FIG. 33, the reference numeral 111 denotes a sensor flag; 112 denotes a photo-sensor ON/OFF of which is controlled as the sensor flag 111 is shifted; and 113 denotes a spring member. Incidentally, since the sensor flag 111 is biased by the spring member 113, if the sheet does not exist in the sensor portion, a condition shown in FIG. 33 is established.
When a sheet (not shown) conveyed in a direction shown by the arrow f between guide plates 115 abuts against the sensor flag 111, the sensor flag 111 is pushed by the sheet to be rocked around a fulcrum 114 in a direction shown by the arrow g. If the sensor flag 111 is rotated down in the direction g in this way, a signal from the photo-sensor 112 is changed, for example, from OFF to ON, with the result that a controlling device (not shown) detects the passage of the sheet.
By the way, when sheets are conveyed continuously, an amount of the sheet interval required for detecting the sheet positively by the sensor 104 of flag type is determined by a time period during which the sensor flag 111 is returned by the spring member 113 when the sensor flag 111 is changed from a condition that the photo-sensor 112 is blocked by the sensor flag to a condition that the photo-sensor is not blocked by the sensor flag, a time period until the flag position is stabled, and a time period until potential of the photo-sensor 112 is stabled.
FIG. 34 is a view showing a construction of a sheet feeding portion 1000 of the conventional sheet conveying apparatus. Sheets S stacked on a sheet stacking portion 1101 are fed and conveyed one by one to a conveying path H1100 by means of a pick-up roller 1102 which is started to be driven by sheet feeding command. The sheets S fed out by the pick-up roller 1102 enter into a nip portion between a retard roller 1103 and a sheet feeding roller 1104 which constitute separating means and feeding means, and, in the nip portion, only an uppermost sheet among the entered sheets is separated, and the separated sheet is conveyed to the downstream conveying path H1100.
When a plurality of sheets are conveyed, the retard roller 1103 is rotated in a direction opposite to the conveying direction by the action of a torque limiter (not shown) so that sheets other than the uppermost sheet are returned to the sheet stacking portion 1101. Incidentally, in FIG. 34, the reference numeral 1105 denotes a sheet end detecting sensor; and 1106 denotes a pair of pull-out rollers.
After a leading end of the fed sheet S is detected by the sheet end detecting sensor 1105, the sheet is transferred to the pair of pull-out rollers 1106 by which the sheet is pulled out from the nip portion between the sheet feeding roller 1104 and the retard roller 1103 and then is conveyed in the downstream direction.
In such a sheet feeding portion 1000, the sheets S being conveyed continuously are conveyed in such a manner that a predetermined or more distance (sheet interval) between a trailing end of a preceding sheet and a leading end of a succeeding sheet is provided to meet sheet conveying requirement from the image forming apparatus thereby to prevent inconvenience due to conveying delay of the sheet S. By detecting such sheet interval by the sheet end detecting sensor 1105, the conveying condition of the sheet S is monitored.
By the way, in some of sheet feeding portions 1000, so-called speed increase control is effected for the purpose that conveying control timings for the sheet become the same at each of sheet feeding stages having different lengths of conveying paths and that an image forming timing for a first sheet (first copy time in a copying machine) is hastened.
In such speed increase control, the sheet end detecting sensor 1105 is used as a pre-registration sensor, and a sheet feeding speed (conveying speed of the sheet obtained by the pick-up roller 1102 and the sheet feeding roller 1104) is maintained as it is, so that, after the sheet is detected by the pre-registration sensor, the sheet is temporarily stopped at a pre-registration stop position (which is effectively positioned behind the pair of pull-out rollers 1106 in order to stable the stop position) and then conveyance is re-started in a timed relationship thereby to realize stable conveyance, and, after the temporal stop, the re-conveying speed is made greater than the sheet feeding speed.
However, even when such control is effected, it is very important that the leading end of the sheet S is detected correctly by the sheet end detecting sensor 1105, and, to this end, it is required that the sheet interval be provided similar to the conveying portion shown in FIG. 32.
Further, in the sheet conveying apparatus, since skew-feed correction for aligning the posture and position of the sheet in front of the image forming portion or the image reading portion must be effected, in the past, various skew-feed correcting means (so-called registration means) have been proposed. Among them, there is means for effecting the skew-feed correction by temporarily stopping the sheet. In such skew-feed correction, greater sheet interval was required.
By the way, in a copying machine as an example of a conventional image forming apparatus of analogue type, even when continuous copying is effected after a single sheet (original) is read, an optical device for exposing the original must be reciprocated by times corresponding to the copy number, and, thus, the sheet interval is inevitably determined.
On the other hand, as image forming apparatuses and image reading apparatuses have been digitalized, by reducing the sheet interval to process the larger number of sheets within short time period, for example in case of image formation, substantial image forming speed has been enhanced without increasing a process speed of the image formation.
The reason is that, since the image reading and image formation are digitalized, after the original was once read, image information can be electrically coded and be stored in a memory, or, in the image formation, the information can be read out from the memory and an image corresponding to the image information of the original can be formed on a photosensitive member by means of an exposing apparatus such as a laser beam, an LED array or the like, with the result that, even when a plurality of sheets are copied, mechanical movement of an optical device and the like can be eliminated.
By the way, nowadays, in the image forming apparatus and the image reading apparatus (referred to as xe2x80x9cimage forming apparatus and the likexe2x80x9d hereinafter), higher image quality and higher productivity have been requested.
For example, in the image forming apparatus, when the high image quality is requested, it is advantageous that the sheet is conveyed at a slow speed in the image forming portion, and, also in case of an image forming apparatus having a fixing portion for fixing the image, it is advantageous that the sheet is conveyed at a slow speed in the fixing portion. However, if the sheet is conveyed at the slow speed in this way, the productivity will be worsened.
Thus, in order to achieve high productivity while maintaining the high image quality, it is required that the distance between the sheets (sheet interval) must be made smaller. However, when a sensor of flag type is used as detecting means for monitoring the sheet being conveyed, as mentioned above, due to the returning time of the flag and the electrical response time of the photo-interrupter, the minimum sheet interval is required, thereby limiting the reduction of the sheet interval.
Incidentally, when an optical sensor such as a sensor of reflection type is used as means for solving this problem, although it is possible to reduce the sheet interval in comparison with the sensor of flag type, the optical sensor is more expensive than the sensor of flag type and cannot be used with a permeable sheet such as an OHP film.
The present invention is made in consideration of the above circumstances, and an object of the present invention is to provide a sheet conveying apparatus which can monitor a sheet conveying condition even with small sheet interval, an image forming apparatus having such a sheet conveying apparatus, an image reading apparatus having such a sheet conveying apparatus, and a sheet processing apparatus having such a sheet conveying apparatus.
The present invention provides a sheet conveying apparatus comprising detecting means for detecting each of the sheets to be conveyed, wherein, when the sheets are conveyed continuously, conveyance of a preceding sheet and a succeeding sheet is started in a condition that the succeeding sheet cannot be detected by the detecting means, and the preceding sheet and the succeeding sheet are conveyed in such a manner that a an interval which can be detected by the detecting means is created between the preceding sheet and the succeeding sheet at a position of the detecting means.